perf: Fix libjvmti.c sign compare error

Fix the compile errors when compiling with -Werror=sign-compare.

This is a follow-up patch to a previous patch series for a separate
issue.  Link: https://lore.kernel.org/lkml/aC9lXhPFcs5fkHWH@x1/



Signed-off-by: Yuzhuo Jing <yuzhuo@google.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250604173632.2362759-1-yuzhuo@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>